Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069, 5,593,838, and 5,603,351, all assigned to the David Sarnoff Research Center, Inc. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analysis", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and them pumped into a bottom array of reaction cells. The array may be decoupled from the assembly and removed for incubation or analysis. When used as a printing device, the chemical reagent solutions are replaced by dispersions of cyan, magenta, and yellow pigment, and the array of reaction cells may be considered a viewable display of picture elements, or pixels, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of the paper fibers pulls the dye from the cells and holds it in the paper, thus producing a paper print, or photograph, of the original scene. One problem with this kind of printer is the accurate control of the print density. The problem comes about because the capillary force of the paper fibers is strong enough to remove all the ink from the device, draining it empty. If the paper is not removed from contact with the ink cells at the correct time, the print density will be too high or too low. Moreover, the correct paper contact time varies with the ambient temperature, making the timing problem more difficult. Yet another problem is that different receivers will take up ink by capillary force at different rates, because of differences in paper fiber size and composition. Therefore, the timing problem will be complicated by requiring different removal times of the receiver when different receivers are used. One solution to this problem is given in the above mentioned copending application U.S. patent application Ser. No. 08/868,416 filed Jun. 3, 1997, where a special paper is employed which will absorb only a limited amount of ink. Nevertheless, it would be cheaper and simpler if plain paper can be employed for this kind of printing, and better still if a variety of papers can be employed as receivers. Another solution to this problem is given in the above mentioned copending application U.S. patent application Ser. No. 08/868,102, filed Jun. 3, 1997 wherein an array of microvalves, each individually addressed, controls the flow of ink to the paper. The complexity of individually addressed valves leads to a high cost printing apparatus.
Pad printing is the subject of many recent journal articles. "Everything you wanted to know about pad printing" recently published in Plastics News International states that "Padprinting is the latest technique for printing on objects that are not flat or that vary in size". Pad printing pads are made out of silicone rubber since it repels many substances, including ink, and because it can be molded into any given shape. In the pad printing process, the pad is brought into contact with a "cliche" that has been flooded with ink. The cliche is typically a thin metal plate into which an impression has been made. By flooding the cliche, ink is left in the impression. The printing process is completed when a silicone pad transfers the ink from the impression on the cliche to the article to be printed. Because the impression in the cliche is fixed, the next cycle of the padprinter will print the exact same image.